In comparison with the vehicles using internal combustion engines, electric vehicles have many benefits such as low noise and low vibration. Generally, while driving the vehicle with the internal combustion engine, the driver can realize the loading condition of the engine and judge whether the operation of the vehicle is normal according to the vibration and noise generated by the engine. Due to the low noise and the low vibration of the electric vehicle, the driver cannot realize the operating condition of the electric vehicle according to the generated vibration and noise. Consequently, the possibility of causing breakdown of the electric vehicle in the overloading or overheating condition will be increased. Although the vehicle body is equipped with instruments, the driver may ignore the messages on the instruments if the traffic condition is crowded or the driving time period is very long.